Weird Cable Problem

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  • #4387
    Joe Mahurin
    Participant

    OK guys, please help me figure this one out.

    I need to lengthen my Z and Y axis so that I can move my hobbycnc pro off the desk with my cnc machine.

    I purchased some 1176C wire and soldered the my current z axis cable with the new wire. When I hook it up the z axis works perfectly but I can’t get the limit stop to clear. I de-soldered and checked the cable for continuity and shorts but it looks fine. So I soldered on some DB9s so I can check the z axis that way, same thing. I took a different piece of 1176C, soldered on a DB9, same problem with the z axis.

    I am an EE so hit me with solutions or probable causes.

    Thanx
    7ofclubs

    #4388
    Joe Mahurin
    Participant

    One other thing, I am using Mach3

    #4389
    BrianV
    Keymaster

    Help me understand.

    You made the existing wiring longer by adding in a length of 6 conductor wire and the z-axis works as it should.

    Can you provide extra detail on what you mean by “I can’t get the limit stop to clear”.

    BrianV

    #4412
    Joe Mahurin
    Participant

    Hi Brian,

    Yes. I added a piece of 1176C to the existing 1176C to lengthen the cable that controls the stepper.

    On Mach3 there is a red button on the control screen marked as Reset. Whenever a limit switch is tripped the Reset button flashes and the CNC will not budge until you press the Reset button. When I had the second wire hooked up to the z axis the limit button would not clear. As soon as I pressed the Reset button it acted like the limit switch was immediately tripped again. Without the second piece of cable attached it all works perfectly.

    I actually solved it by removing the 1st piece of cable and soldering on the second longer piece of cable directly to the stepper motor. I am at a loss to explain why you can’t solder an addition length of cable to the stepper.

    7ofclubs

    #4414
    Joe Mahurin
    Participant

    OK, cancel that last post, it still doesn’t work.

    I so hooked up the z and y axis using that new cable and removed the short piece. I can get the limit switch Reset button to clear the limit but as soon as the z axis starts moving it trips the limit, even if the CNC machine is no where near tripping the physical switch. Does the same thing on the y axis also. The x axis works perfectly.

    Could their be a problem with my HobbyCNC pro board? I bought this thing in ~2007.

    7ofclubs

    #4423
    BrianV
    Keymaster

    It’s highly unlikely it is the HobbyCNC board. The pins for limit switches go straight from the screw-terminal connector to the 25 Pin D. There is a small resistor network to provide 10k Ohms pull-up to +5 Volts.

    10k is a pretty high impedance. It is possible that by fiddling with your wiring, you somehow relocated your limit switch wiring such that it is affected by the large inductive spikes when the stepper motors are turned ‘off’. You might be getting enough noise radiated into your limit switch wiring.

    If you disconnect the limit switches at the HCNC board, does the problem go away? Disconnect at one of the numbered screw terminals (10, 11,12,13,15), AND at the GND pin. (of course, this just substitutes a potential travel-past-limits problem).

    Your limit switch circuit is not getting any undesired contact from any other wiring at any point?

    Do you have a break in continuity in your limit switch string? If I don’t hook mine up, the unit will throw all kinds of errors the instant I hit “go”. I did also have one intermittent limit switch that would randomly go open.

    I do have an optoisolator between my limit switches and my PRO board – the thought of the dirty “outside world” power potentially being sent directly into my Parallel Port scares the hell out of me. You can check out my set-up here: WoodWorkerB Setup Configuration Limit Switches

    FWIW, I made that PC board on my mill.

    How, exactly is your limit switch wiring run – close to the stepper motor wiring? Any optoisolation? Is your limit switch wiring COMPLETELY separate from all other wiring/circuitry/voltage?

    BrianV

    #4428
    Joe Mahurin
    Participant

    Call me an idiot. I’m basically a low voltage, milliamp, microcontroller type of guy. Parasitic inductance never occurred to me because of my background.

    Yeah, so call me a moron. I have my limit switch wiring parallel to my y and z cables for at least 3 feet, I have them run through a plastic chain wire guide, unshielded.

    Anyone got a skinny faraday cage I can use?

    7ofclubs

    #4442
    BrianV
    Keymaster

    I’m a Microchip PIC guy, myself.

    So, do you think you have a grip on the issue? Today I milled an optoisolator PCB like the one I use. I run all my wiring right next to each other, never had a problem.

    BrianV

    #4499
    Joe Mahurin
    Participant

    Yeah, thanx. I got a handle on it. I don’t know if I will just do an opto isolator or invest in some shielded cable.

    Do you have the circuit board file you are milling or is it on the site you mentioned?

    Wesley

    #4568
    BrianV
    Keymaster

    I can make the files for the opto board available, if you wish.

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